What you describe for getting the airplane back into service, Jim, is the “way it was.” With new operating limitations the rules have changed. An airplane owner with the newer version of the limitations doesn't have to deal with the FAA.<br /><br />Here’s what my current (2001) operating limitations say:<br /><br />“After incorporating a major change as described in Section 21.93 the aircraft owner is required to re-establish compliance with FAR 91.319(b). All operations will be conducted day VFR in a sparsely populated area. The aircraft must remain in flight test for a minimum of 5 hours. Persons non-essential to the flight shall not be carried. The aircraft owner shall make a detailed log book entry describing the change prior to the test flight. Following satisfactory completion of the required number of flight hours in the flight test area, the pilot shall certify in the records that the aircraft has been shown to comply with FAR 91.319(b). Compliance with FAR 91.319(b) shall be recorded in the aircraft records with the following or a similarly worded statement: (I won’t bore you any further with the exact wording.)<br /><br />Earlier versions of the operating limitations required that the aircraft be re-inspected as you indicated.<br /><br />Note that FAR 91.319(b) only pertains to hazardous flight characteristics. (§91.319 Aircraft having experimental certificates: Operating limitations. (b) No person may operate an aircraft that has an experimental certificate outside of an area assigned by the Administrator until it is shown that -- (1) The aircraft is controllable throughout its normal range of speeds and throughout all the maneuvers to be executed; and (2) The aircraft has no hazardous operating characteristics or design features.)<br /><br />The definition of a major modification can be found in FAR 21.93. This is what it says: § 21.93 Classification of changes in type design. (a) In addition to changes in type design specified in paragraph (b) of this section, changes in type design are classified as minor and major. A 'minor change' is one that has no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product. All other changes are 'major changes' (except as provided in paragraph (b) of this section).<br /><br />It is obvious that changing from a 912 to a 912S would be a major modification.<br /><br />There is another provision that relates to operations after maintenance. It is FAR 91.407:<br /><br />§91.407 Operation after maintenance, preventive maintenance, rebuilding, or alteration. <br />(a) No person may operate any aircraft that has undergone maintenance, preventive maintenance, rebuilding, or alteration unless -- <br />(1) It has been approved for return to service by a person authorized under §43.7 of this chapter; and <br />(2) The maintenance record entry required by §43.9 or §43.11, as applicable, of this chapter has been made. <br />(b) No person may carry any person (other than crewmembers) in an aircraft that has been maintained, rebuilt, or altered in a manner that may have appreciably changed its flight characteristics or substantially affected its operation in flight until an appropriately rated pilot with at least a private pilot certificate flies the aircraft, makes an operational check of the maintenance performed or alteration made, and logs the flight in the aircraft records. <br />(c) The aircraft does not have to be flown as required by paragraph (b) of this section if, prior to flight, ground tests, inspection, or both show conclusively that the maintenance, preventive maintenance, rebuilding, or alteration has not appreciably changed the flight characteristics or substantially affected the flight operation of the aircraft. <br /><br />Subsection (c) indicates that ground tests or inspection or both can prove there has been no major change in flight characteristics. From the history that has now been developed I think it can be argued that changing the prop from an IVO to a Warp Drive would be a minor change and approved on the basis of a “ground test” or inspection. Caveat Emptor: I don’t know that for a fact.<br /><br />